Falls are the second leading cause of accidental injury deaths worldwide; there are approximately 684,000 fatal falls in the world every year, more than 80% of which happen in low- and middle-income countries, usually involving people who are older and more dependent [1]. More than 2.8 million people are treated in emergency departments because of falls every year, and a quarter of falls cause serious injuries, such as fractures or brain injury [2]. Furthermore, older adults who have experienced falls suffer from increased anxiety about falling, physical immobility and loss of independence [3].

Increased rates of falls and fall-related injuries are not the consequence of a single cause but rather the outcome of a range of distinct risk factors, including advancing age, cognitive decline, certain chronic diseases, female gender, medication usage, and environment [4–7]. The underlying biological mechanisms involved in falls and fall-related injuries due to environmental problems remain ill-defined. Studies in animals showed that exposure to biodiesel fuels changed the expression of genes related to antioxidant defense and inflammation [8], which can cause falls [9]. Besides, the study found an increase in glial-fibrillary acidic protein (GFAP) in the corpus callosum of mice exposed to diesel particulate material [10], and long-term exposure to air pollution impaired cognitive function [11–13]. In human studies, it was found that healthy individuals exposed to high levels of air pollutants showed ultrafine particles in the olfactory bulb and α-prominent nuclear proteins in neurons and glial cells, causing a decline in cognitive function [14]. Moreover, it can be seen that air pollution might affect the cognitive function of nervous system, which has close connections with falls [15–18]. Although evidence for a relationship between air pollutants and physical limitations is growing, reliable information on solid fuels and the onset of falls and fall-related injuries is still limited.

A cross-sectional study from China of 12,458 adults reported that solid fuels for cooking may be associated with self-reported and performance-based declines in physical functioning in middle-aged and older populations [19], and a longitudinal study from the Netherlands of 1,763 adults aged 55–85 years reported that performance-based physical functioning decreased with increasing levels of air pollutants [20]. Another nationwide longitudinal study in China, with 10,832 participants (65–105 years of age), concluded that an increase in the annual average concentration of fine particles of 10 μg/m³ is accompanied by a 5% reduction in balance ability and that fine particles are also separately associated with various other physical limitations [21].

Based on the China Health and Retirement Longitudinal Study (CHARLS), this study analyzes the impact of household solid fuel use on subsequent falls and fall-related injuries in order to provide a reference for fall prevention and injury reduction among older adults and to improve quality of life in an aging society.

Falls and fall-related injuries are notable problems in the healthcare industry, especially for patients and older people. Risk factors for falls include muscle weakness, agitation, confusion, postural hypotension, and sedative medication [30]. Recent studies have shown that environmental pollutants such as PM2.5 are also risk factors for falls and fall-related injuries [31]. Short-term and long-term exposure to environmental pollutants may be associated with a series of adverse health outcomes of physical function, such as slower gait, balance disorders, and tremors [14, 21]. Moreover, some studies have shown the relationships between the indoor use of solid fuels and respiratory diseases, cardiovascular diseases, and cognitive dysfunction in different regions [32–34]. Although outdoor air pollution has been linked to falls, but limited studies have investigated the relationship between household air pollution of solid fuels and falls and fall-related injuries in adults. Therefore, we analyzed data from the China Health and Retirement Survey (CHARLS), a national survey of about 17,000 residents aged over 45 to explore the associations between solid fuels with falls and fall-related injuries.

In this large, national, prospective cohort study in China, we found that when the covariates were adjusted—gender, age education level, marital status, living residence, economic situation, smoking status, drinking status, body mass index, physical activity, self-reported previous diseases including hypertension, diabetes, stroke, psychiatric problems and memory-related diseases and baseline falls history—the use of solid fuels for cooking and heating at baseline increased the risks of falls and fall-related injuries.

Our findings showed that solid fuels used for cooking were significantly correlated with falls and fall-related injuries among adults over 45 years old, which possibly suggested that solid fuels might be a risk factor for falls and fall-related injuries. These are consistent with the results of several previous studies on the decline of physical functions of participants exposed to outdoor air pollution. A study from Netherlands suggests that exposure to air pollution may adversely affect physical performance of older adults [20]. Similarly, another prospective cohort study with 6,157 participants (≥65 years of age) from Chicago, Illinois, USA indicated that long-term exposure to nitrogen oxides (NOx) may be associated with a faster aging-related decline in physical functioning [35].

In addition, the proportion of solid fuels used for heating is over 70% in Table 1, which is consistent with the increase in the stocking of domestic fuel for winter heating in northern China during the same period [36]. However, our results suggested that in the full model, solid fuels used for heating was significantly correlated with falls rather than fall-related injuries among adults over 45 years old. Such results may be caused by a large number of exposure misclassification caused by factors such as ventilation, emissions from neighbor stove and habits [37]. Additionally, due to concerns about carbon monoxide poisoning, people often pay more attentions to ventilation when heating than when cooking, potentially reducing the levels of pollutants produced by heating fuel use [4].To analyze whether a higher proportion of use of solid fuels induces more falls and fall-related injuries and to study the combined effect of solid fuels used for cooking and heating on falls and fall-related injuries, we combined the data for fuels used for cooking with those used for heating and divided the participants into three categories: the users of clean fuels, the users of mixed fuels, and the users of solid fuels. The results suggested that indoor users of combined cooking and heating solid fuels almost had more falls rather than fall-related injuries. Dual exposure to indoor air pollution from cooking and heating may result in overlapping effects and constitute a persistent danger to human health [23]. These results might provide new knowledge about the impact of solid fuel use on humans that includes depression, sleep disturbance, arthritis, COPD, and so on [5, 23, 38, 39].

Subgroup analysis suggested that the associations between solid fuels with falls and fall-related injuries were significant in almost all different subgroups categorized by sex and residence. This finding confirmed our main results. Compared with these previous studies, this study not only had much larger sample size over a wider age range (≥45 years old at baseline) and a longer follow-up time, but also paid attention to the type of solid fuel use on falls and injurious falls. The underlying biological mechanisms involved in the increased likelihood of falls and fall-related injuries due to household solid fuel use remain ill defined. Numerous risk factors for falls such as psychomotor retardation (including, cognitive problems, dementia, and so on) and gait alteration are common in older people those who were exposure in air pollution [40]. Individuals who were suffered in air pollution also tend to exhibit cognitive deficits [11] that affect attention, executive functions, and processing speed [15, 41], which can all increase the risk of falls and injurious falls. Prior studies supported that people with diabetes were at a significantly higher risk of falls and injurious falls than people without diabetes across [42]. Huiyu Wang et al. found that each 10 μg/m3 increase in annual averaged concentrations of PM2.5 was associated with a 5% risk of reduced balance ability [21]. In addition, some experiments have demonstrated the adverse effects of solid fuels or indoor air pollution on physical health. A cross-sectional study showed that household use of solid fuels is also a risk factor for the development of cognitive impairment [4]. Also, exposure to indoor PM2.5 can increase IL-7 levels in C57 mice [43], and elevated IL-7 levels increased the likelihood of single and recurrent falls [44]. In general, the adverse effect of solid fuels on health might aggravate the diseases that induce falls and fall-related injuries.

Our study has several strengths. First, the main strength of the present study is that it is a national population-based sample, providing rich data to support the reliability of our conclusions. Second, our study simultaneously investigated the relationship between solid fuels and falls and fall-related injuries, and prospectively assessed the correlation. Finally, the national representation of CHARLS is widely recognized, so our results can be used not only as evidence for fall prevention among older people in China, but also as a reference for future research in other countries, especially in developing countries.

However, there are also some limitations in this study. First, the current study adopted modified Poisson regression rather than survival analysis because falls are investigated every 2 years, and we cannot obtain accurate exposure time from the questionnaire. Second, despite controlling for many potential covariates, residual confounding may have influenced our observed relationships. Third, information collected from participants was based on self-report, so this study cannot rule out the possibility of recall bias. Previous research consistently showed that older adults tended to under-report falls because they did not recognize the severity of a fall or did not remember a fall with less severe consequences [45]. Therefore, there might be recall bias. Fourth, CHARLS has amounts of missing or incomplete data. Incomplete datasets may lead to potential biases. Finally, due to the true quantitative value of pollutant concentration is not involved in this questionnaire, we can’t directly explore the relationship between the real value caused by household air pollution with falls and fall-related injuries in middle-aged and older people. Although the type of fuels has adopted to replace it, which greatly weakens the causality of this study. It is expected that subsequent studies will be conducted on pollutant types or specific concentrations. However, we hope this study may generate people’s interest in gaining a better understanding of the relationships between solid fuels and falls or fall-related injuries.